Image-capturing lens and image-capturing apparatus

ABSTRACT

An image-capturing lens includes a plastic lens formed with use of a material in which particles each having the maximum length of 30 nanometer or less are dispersed in a plastic material.

FIELD OF THE INVENTION

The present invention relates to an image-capturing lens of animage-capturing apparatus employing a solid image sensor such as animage sensor of a CCD (Charge Coupled Device) type or an image sensor ofa CMOS (Complementary Metal-Oxide Semiconductor) type, and inparticular, to an image-capturing lens wherein fluctuations in an imagepoint position caused by temperature changes are small and to animage-capturing apparatus employing the image-capturing lens.

The present invention further relates to a zoom lens whereinfluctuations in an image point position caused by temperature changesare small in spite of a plastic lens and to an image-capturing apparatusemploying the zoom lens.

RELATED ART

In recent years, a cellphone having a photographing function has beenpopularized rapidly, and under the influence of downsizing of animage-capturing apparatus using a solid image sensor such as an imagesensor of a CCD type or an image sensor of a CMOS type which can behoused in the cellphone, demands for downsizing of an image-capturinglens to be housed have been increased. Therefore, a focal length of thetotal system of the image-capturing lens has been required to be small,which also has requested a radius of curvature and an outside diameterof each lens to be small, thus, it has become difficult for a glass lensmanufactured through grinding and polishing to be processed, and aplastic lens which can be manufactured at low cost on a mass-productionbasis has come to be used. Patent Document 1 discloses a conventionalexample wherein all lenses constituting an image-capturing lens areplastic lenses.

(Patent Document 1) U.S. Pat. No. 3,396,683

However, the lens disclosed in the Patent Document 1 has a problem thata change in an image point position caused by a change of refractiveindex of plastic material resulted from temperature changes isrelatively large. In the case of a small-sized image-capturing apparatussuch as one housed in a cellphone, in particular, the so-calledpan-focus system having no autofocusing mechanism of a lens is usuallyemployed in many cases, and under such image-capturing apparatus, achange in an image point position caused by temperature changes cannotbe ignored, which causes a fear of out-of-focus images in photographingunder the severe temperature environment. For this problem, it ispossible to provide an autofocusing mechanism on the image-capturingapparatus, which, however, incurs a fear of the essential problem thatthe cellphone becomes heavy and bulky to lose portability.

In view of the problems stated above, the first object of the inventionis to provide an image-capturing lens wherein fluctuations in an imagepoint position caused by temperature changes are small in spite of aplastic lens and an image-capturing apparatus employing that lens.

Further, in recent years, a digital camera employing a solid imagesensor such as an image sensor of a CCD type or an image sensor of aCMOS type has been popularized rapidly. Under this condition, there aredemands for a digital camera which is highly efficient and is low inprice. For the high efficiency, the main trend is to improve aberrationcharacteristics by using many glass mold aspheric lenses. However, theglass mold aspheric lens is difficult to be processed and is high inprice, which causes a problem that a price of the zoom lens is extremelyhigh. Therefore, plastic lenses manufactured at low cost on amass-production basis have come to be used. Patent Document 1 disclosesa conventional example wherein many plastic lenses are used.

(Patent Document 2) TOKKAI No. 2003-50352

However, the lens disclosed in Patent Document 2 has a problem thatchanges in a refractive index of plastic material are caused by changesof ambient temperatures, and a change in an image point position becomeslarge relatively, because many plastic materials are used. In contrastto this, in the digital camera employing a recent zoom lens, anautofocusing mechanism is usually housed. Therefore, it can beconsidered to use the autofocusing mechanism to drive the zoom lens toabsorb changes of refractive index of the lens caused by temperaturechanges.

However, when setting the mode for fixing a focus to the hyperfocaldistance without using autofocusing, or when driving a lens forcibly toany one of plural zoom positions determined in advance, like theso-called step zoom, if fluctuations of image point positions in thecase of temperature changes are great, the mechanism to correct thefluctuations needs to be provided, thereby, a zoom mechanism needs to becomplicated, which is a problem. Further, when using a plastic lens fora zoom lens, there have been various restrictions, including that theplastic lens cannot be used for the lens having a large refractingpower, for reducing fluctuations of image point positions, or that thelens structure is determined in advance.

In view of the problems stated above, the second object of the inventionis to provide a zoom lens wherein fluctuations in an image pointposition caused by temperature changes are small in spite of a plasticlens and an image-capturing apparatus employing that lens.

DISCLOSURE OF THE INVENTION

Since the image-capturing lens in Item 1-1 attaining the first objecthas a plastic lens formed by using the material wherein particles eachhaving the maximum length of not more than 30 nanometers are dispersed,it is possible to provide an image-capturing lens having excellentoptical characteristics in spite of environmental changes whereinfluctuations in image point positions are restrained, and lighttransmittance is not lowered, by dispersing particles each having themaximum length of not more than 30 nanometers in the plastic materials.

The zoom lens in Item 2-1 attaining the second object has a plastic lensformed by using the material wherein particles each having the maximumlength of not more than 30 nanometers are dispersed in plastic material,in the zoom lens composed of plural lens groups wherein magnification isvaried by changing a distance between the lens groups.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an image-capturing apparatus includingan image-capturing lens.

FIG. 2 is a sectional view of an image-capturing lens in Second Example.

FIG. 3 is a sectional view of an image-capturing lens in Third Example.

FIG. 4 is a sectional view in the optical axis direction of animage-capturing apparatus including a zoom lens in each of FourthExample and Fifth Example.

FIG. 5 is a sectional view in the optical axis direction of animage-capturing apparatus including a zoom lens in Sixth Example.

FIG. 6 is a sectional view in the optical axis direction of animage-capturing apparatus including a zoom lens in Seventh Example.

PREFERRED EMBODIMENT OF TEE INVENTION

A basic concept of the invention will be explained first. In general,when fine powder is mixed with transparent resin material, lightscattering is caused and transmittance is lowered, thus, it has beendifficult to use them as optical materials. However, it has been clearedthat light scattering can be prevented substantially, by making a sizeof fine power to be smaller than a wavelength of transmitted light flux.

Further, though a refractive index of a plastic material is lowered whena temperature of the plastic material rises, a refractive index of aninorganic particle is raised when a temperature of the inorganicparticle rises. Therefore, it is possible to arrange so that refractiveindex changes are hardly caused, by utilizing thesetemperature-dependencies in a way that the temperature-dependenciesoffset each other. Specifically, it is possible to provide a materialhaving totally an extremely low temperature-dependency, by dispersing,in plastic materials representing base materials, some inorganicparticles whose maximum particle edge length is not more than 30nanometers, preferably is not more than 20 nanometers, and morepreferably is in a range of 10-15 nanometers.

For example, it is possible to make a change of refractive index for thetemperature of this kind to be small, by dispersing fine-particles ofniobium oxide (Nb₂O₅) in acrylic resins, and thereby to controleffectively a change in an image point position resulting fromtemperature changes.

The preferred structure to attain the first object will be explained asfollows.

Since the plastic lens formed by using materials wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed in plastic material is a positive plastic lens, it is possibleto restrain effectively changes in image point position of animage-capturing lens which are caused by temperature changes, in theimage-capturing lens in Item 1-2.

Since the image-capturing lens in Item 1-3 is composed of at least twolenses in the image-capturing lens in Item 1-1, it is possible tocorrect various aberrations properly, compared with an image-capturinglens of a single-piece structure, by making at least the image-capturinglens to be the image-capturing lens composed of two lenses.

Since the image-capturing lens in Item 1-4 has at least two plasticlenses in the image-capturing lens described in Item 1-1, and theplastic lens having the strongest refracting power among the plasticlenses is the plastic lens formed by using a material wherein particleswhose maximum particle edge length is not more than 30 nanometers aredispersed in plastic material, it is possible to restrain moreeffectively the change in image point position of the image-capturinglens caused by the temperature changes.

The image-capturing lens in Item 1-5 has at least a positive lens and anegative lens in the image-capturing lens described in Item 1-1, and thepositive plastic lens is a plastic lens formed by using a materialwherein particles whose maximum particle edge length is not more than 30nanometers are dispersed in plastic material. Since it is easy to add anaspheric surface to a plastic lens, it is possible to add easily anaspheric surface to a positive lens and a negative lens in theimage-capturing lens, by providing an image-capturing lens having atleast a positive lens and a negative lens, thus, various aberrations inthe total system of the image-capturing lens can be corrected properly.Further, if the positive plastic lens is made to be a plastic lensformed by using a material wherein particles whose maximum particle edgelength is not more than 30 nanometers are dispersed in plastic material,changes in image point positions of the image-capturing lens resultedfrom temperature changes can be restrained effectively, while correctingvarious aberrations of the total image-capturing lens systemeffectively.

The image-capturing lens of Item 1-6 is one wherein the image-capturinglens described in Item 1-1 is totally composed of plastic lenses, and atleast one plastic lens is one formed by using a material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed in plastic material. Therefore, by making alllenses constituting the image-capturing lens to be a plastic lens, it ispossible to attain weight reduction compared with the image-capturinglens including a glass lens. Furthermore, if at least one lens is madeto be a plastic lens formed by using a material wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed, changes in image point positions resulted from temperaturechanges in the total system of the image-capturing lens can becontrolled.

The image-capturing lens of Item 1-7 is one wherein the image-capturinglens described in Item 1-1 includes at least one glass lens. The numberof types of plastic materials used for lenses are less than those ofglass materials, and refractive indexes and values of dispersion tend tobe limited. Therefore, if at least one glass lens is included in theimage-capturing lens, aberration corrections which are more excellentcan be conducted, because the degree of freedom of selection for therefractive index and dispersion is increased.

The image-capturing lens of Item 1-8 is one according to theimage-capturing lens described in any one of Item 1-1 to Item 1-7wherein an aperture stop is provided, and at least one of a plastic lensthat is closest to the aperture stop among plastic lenses and a plasticlens that is further adjacent to the plastic lens that is closest to theaperture stop is a plastic lens formed by using a material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed in a plastic material, and therefore, it ispossible to control effectively the changes in image point positions ofthe image-capturing lens resulted from temperature changes, by usingsuch material for at least one of the plastic lens closest to theaperture stop and the plastic lens that is further adjacent to theaforesaid plastic lens closest to the aperture stop. Namely, whenchanges in refractive index resulted from temperature changes are causedon the lens near the aperture stop, changes in image point fluctuationsare caused and optical efficiency of the total image-capturing lens isdeteriorated. However, the deterioration of the optical efficiency canbe controlled to be small, by using the aforesaid material. Since it isfurther possible to make a diameter of the lens near the aperture stopto be small, a lens that may be formed easily can be made even from amaterial wherein particles in a nanometer size are dispersed in aplastic material.

The image-capturing lens of Item 1-9 is represented by theimage-capturing lens described in any one of Item 1-1 to Item 1-8wherein the plastic lens formed by using a material wherein particles insize of 30 nanometers or less are dispersed satisfies the followingcondition;|A|<8×10⁻⁵/° C.   (1)

wherein, A represents a change of refractive index caused by atemperature change which is expressed by the following expression.$\begin{matrix}{A = {\frac{( {n^{2} + 2} )( {n^{2} - 1} )}{6n}\{ ( {{{- 3}a} + {\frac{1}{\lbrack R\rbrack}\frac{\partial\lbrack R\rbrack}{\partial t}}} ) \}}} & ( {{Numeral}\quad 1} )\end{matrix}$a. α: Coefficient of linear expansion, [R]: Molecular refraction

The image-capturing lens of Item 1-10 is represented by theimage-capturing lens described in Item 1-9, wherein the plastic lensformed by using a material wherein particles in size of 30 nanometers orless are dispersed satisfies the following condition;|A|<6×10⁻⁵/° C.   (2)

Next, a change of refractive index caused by a temperature change willbe explained. The change of refractive index caused by a temperaturechange A is expressed by the aforesaid Numeral 2 by differentiatingrefractive index n with temperature t based on Lorentz-Lorenz equation.

In the case of a plastic material, a contribution of the second term isgenerally small compared with the first term in the equation, and it canbe ignored substantially. For example, in the case of PMMA resin, thecoefficient of linear expansion α is 7×10⁻⁵, and when it is substitutedin the aforesaid expression, A=−1.2×10⁻⁴/° C. holds, which agreessubstantially with an actual measurement.

In the invention, in this case, a contribution of the second term in theaforesaid expression is made to be great substantially by dispersingfine particles, preferably inorganic fine particles, in a plasticmaterial, so that changes caused by linear expansion of the first termand those caused by the second term may cancel each other.

Specifically, it is preferable that the change of refractive indexcaused by a temperature change A which has been about −1.2×10⁻⁴/° C. inthe past is controlled to be less than 8×10⁻⁵/° C. on an absolute valuebasis. It is further preferable to control to be less than 6×10⁻⁵/° C.on an absolute value basis. It is more preferable to control to be lessthan 4×10⁻⁵ on an absolute value basis.

It is further possible to make a contribution of the second term to begreater so that there may be provided temperature characteristics whichare opposite to those of the plastic material representing a basematerial. In other words, it is also possible to obtain a material whoserefractive index is raised without being lowered when a temperaturerises.

With respect to the rate of mixing, 80 is for the plastic materialrepresenting a base material and about 20 is for niobium oxide on avolume ratio basis, and these are mixed evenly. Though fine particleshave a problem to tend to cohere, necessary state of dispersion can beobtained because there is known a technology to disperse by givingcharges to the surface of each particle.

Incidentally, this volume ratio can be raised or lowered properly forcontrolling a rate of change of refractive index for temperatures, andplural types of inorganic particles in nanometer sizes can be blendedand dispersed.

Table 1 shows changes of refractive index of plastic material applicableto the invention caused by a temperature change A (=dn/dT) TABLE 1Plastic material A (Approximate value) 10⁻⁵/° C. Acryl-based −12Polycarbonate-based −14 Polyolefin-based −12 Polyester-based −14

Table 2 shows changes of refractive index of inorganic materialapplicable to the invention caused by a temperature change A (=dn/dT)wherein a direction of a symbol is different from the plastic materialTABLE 2 Inorganic material A (Approximate value) 10⁻⁵/° C. Aluminumoxide 1.4 ALON 1.2 Beryllium oxide 1.0 Diamond 1.0 Calcium carbonate 0.7Titanium potassium phosphate 1.2 Magnesium aluminate 0.9 Magnesium oxide1.9 Quartz 1.2 Tellurium oxide 0.9 Yttrium oxide 0.8 Zinc oxide 4.9

The image-capturing lens of Item 1-11 is represented by theimage-capturing lens described in any one of Item 1-1 to Item 1-10,wherein the particles are inorganic materials.

The image-capturing lens of Item 1-12 is represented by theimage-capturing lens described in Item 1-11, wherein the inorganicmaterials are oxides.

The image-capturing lens of Item 1-13 is represented by theimage-capturing lens described in Item 1-12, wherein the oxides are inthe state of saturated oxidation.

It is preferable that fine particles are an inorganic substance and itis more preferable that they are an oxide. Oxides which are saturated interms of the state of oxidation and are not oxidized any more arepreferable. Fine particle which is an inorganic substance is preferablebecause its reaction with a plastic material representing a high polymerorganic compound can be controlled to be low, and its deteriorationcaused when it is used can be prevented when it is an oxide. Further, itis naturally possible to prevent oxidization of the plastic material byadding antioxidants.

The image-capturing lens of Item 1-14 is represented by theimage-capturing lens described in any one of Item 1-1 to Item 1-13,wherein a volume ratio between the plastic material and the particlesdispersed in the plastic material is within a range of 9:1-3:2.

Though the volume ratio is 80:20, namely, 4:1 in the aforesaid example,it can be adjusted properly within a range of 90:10 (9:1)-60:40 (3:2).If an amount of the particles is less, exceeding the ratio of 9:1, aneffect to restrain a change by temperature turns out to be smaller,while, when the ratio of 3:2 is exceeded, there is caused a problem inmoldability, which is not preferable. It is more preferable to adjust adegree of refractive index change in the material wherein fine particlesare dispersed in a plastic material, by considering an influence on thetotal image-capturing lens by temperature changes.

The image-capturing lens of Item 1-15 has therein the image-capturinglens described in any one of Item 1-1 to Item 1-14.

The invention makes it possible to provide an image-capturing lenswherein a fluctuation of image point position in the case of temperaturechanges is small even when a plastic lens is used, and to provide animage-capturing apparatus employing the image-capturing lens.

Referring to the drawings, there will be explained in detail theembodiment of the invention for attaining the first object of theinvention to which, however, the invention is not limited. Incidentally,“a plastic lens” in the invention includes a lens wherein a basematerial is a plastic material, and particles each being small indiameter are dispersed in the plastic material and a volume ratio ofplastic is one half or more, and it also includes a lens whose surfaceis subjected to coating processing for the purpose of antireflection andan improvement of surface hardness.

FIG. 1 is a sectional view in the optical axis direction of animage-capturing apparatus including an image-capturing lens relating tothe present embodiment. In FIG. 1, the image-capturing lens includesaperture stop S, first lens L1, second lens L2 and infrared cut filterIRCF in this order from the object side, and the image-capturingapparatus is composed of the image-capturing lens and of a solid-stateimage sensor such as image sensor CMOS arranged on the image side of theinfrared cut filter IRCF. An optical image that is formed onimage-capturing surface I by passing of light through theimage-capturing lens is converted into electric signals by image sensorCMOS, and is further subjected to prescribed processing, to be convertedinto electric signals.

The first lens L1 adjoining the aperture stop S is a plastic lens thathas the greatest refracting power and is formed by using a materialwherein particles whose maximum particle edge length is not more than 30nanometers are dispersed in a plastic material. The second lens L2 is aplastic lens containing no particles. However, there are consideredother various structures in addition to the foregoing. Though the numberof lenses is two in the present embodiment, three lenses or not lessthan four lenses may also be used, and in that case, one or both of thelens adjoining the aperture stop S and the lens adjoining the aforesaidlens may be made to be a plastic lens formed by using a material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed.

Now, a preferred example in the present embodiment will be explained asfollows. Symbols used in the example are as follows.

F: F-number

2Y: Length of diagonal line on image-capturing surface of solid-stateimage sensor (length of a diagonal line of the rectangular effectivepixel area of solid-state image sensor)

R: Curvature radius of refractive surface

D: Distance between refractive surfaces on axis

Nd: Refractive index of lens material for d line at a normal temperature

υd: Abbe's number of lens material

f: Focal length

fB: Back focus

In the present embodiment, a shape of an aspheric surface is expressedby “Numeral 3”, in the rectangular coordinates wherein the origin isrepresented by the vertex of the surface and the horizontal axis isrepresented by the optical axis direction, under the conditions that Crepresents the curvature of the vertex, K represents the conic constantand A4, A6, A8, A10 and A12 represent the aspheric surface coefficients.$\begin{matrix}{X = {\frac{{Ch}^{2}}{1 + \sqrt{1 - {( {1 + K} )C^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}} + {A_{8}h^{8}} + {A_{10}h^{10}} + {A_{12}h^{12}}}} & ( {{Numeral}\quad 2} )\end{matrix}$wherein, h=√{square root over (Y²+Z²)}

Tables 3 and 4 show lens data related to the present embodiment.Incidentally, in the Tables, the exponentiation of 10 (for example,2.5×10⁻³) is to be expressed by using E (for example, 2.5×E-3). TABLE 3(Example 1) f = 2.13 mm   fB = 0.41 mm   F = 2.88   2Y = 3.00 mm SurfaceNo. R (mm) D (mm) Nd μd Aperture ∞ 0.05 1 3.433 1.10 1.53175 56.0 2−0.774 0.59 3 −0.347 0.50 1.58300 30.0 4 −0.568 0.30 5 ∞ 0.30 1.5163364.1 6 ∞

TABLE 4 Aspheric surface coefficient First surface K = 4.60380 × E+01 A4= −4.84740 × E−01 A6 = 2.07620 × E+00 A8 = −3.83570 × E+01 A10 = 2.13660× E+02 A12 = −5.45620 × E+02 Second surface K = −2.18110 × E−01 A4 =1.85600 × E−01 A6 = −6.82460 × E−01 A8 = 1.55450 × E+00 A10 = −2.03740 ×E+00 A12 = 9.69370 × E−01 Third surface K = −8.57310 × E−01 A4 = 1.21860× E+00 A6 = −1.32960 × E+00 A8 = 6.34950 × E+00 A10 = −1.40400 × E+01A12 = 1.23400 × E+01 Fourth surface K = −9.29500 × E−01 A4 = 4.97610 ×E−01 A6 = −2.36140 × E−01 A8 = 6.61190 × E−01 A10 = −7.94890 × E−01 A12= 3.27600 × E−01

FIRST EXAMPLE

The present example is a design example corresponding to theimage-capturing lens in FIG. 1 wherein aperture stop S is arranged to beclosest to an object, and infrared cut filter IRCF is arranged to beclosest to an image. First lens L1 is a polyolefin-based plastic lensformed by using the material wherein particles each having a diameter ofnot more than 30 nanometers are dispersed in plastic material, andsecond lens L2 is a polycarbonate-based plastic lens containing noparticles.

Changes of refractive index caused by temperatures are shown in Table 3.According to this, an amount of changes (ΔfB) of back focus in the caseof temperature rise of +30° C. from the normal temperature (20° C.) is+0.010 mm when A=−8×10⁻⁵/° C. holds for the first lens L1, and it is+0.006 mm when A=−6×10⁵/° C. holds for the first lens L1, while, anamount of changes (ΔfB) of back focus in the case of temperature fall of−30° C. is −0.010 mm when A=−8×10⁻⁵/° C. holds for the first lens L1,and it is −0.006 mm when A=−6×10⁻⁵/° C. holds for the first lens L1.TABLE 5 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. First −8 × 10⁻⁵ 1.5318 1.5294 1.5342 lens −6 × 10⁻⁵ 1.53001.5336 Second −14 × 10⁻⁵  1.5830 1.5788 1.5872 lens

SECOND EXAMPLE

FIG. 2 is a sectional view of an image-capturing lens of Second Example.Tables 6 and 7 show lens data related to the present example. TABLE 6(Example 2) f = 3.812 mm   fB = 0.846 mm   F = 2.88   2Y = 4.61 mmSurface No. R(mm) D(mm) Nd μd 1 1.818 1.00 1.53180 56.0 2 3.713 0.25Aperture ∞ 0.62 3 −1.687 1.22 1.53180 56.0 4 −0.900 0.10 5 6.800 0.781.58300 30.0 6 1.534

TABLE 7 First surface K = 9.1572 × E−01 A4 = −3.8570 × E−03 A6 = 1.1925× E−03 A8 = 1.5434 × E−03 A10 = −1.0585 × E−03 Second surface K = 1.9004× E+01 A4 = 1.4293 × E−03 A6 = 6.6467 × E−02 A8 = −1.0545 × E−01 A10 =−2.1010 × E−02 Third surface K = −1.9422 × E−01 A4 = −2.5670 × E−02 A6 =−2.3520 × E−01 A8 = 3.4025 × E−01 A10 = −7.1481 × E−02 Fourth surface K= −2.8233 × E+00 A4 = −2.1793 × E−01 A6 = 1.3119 × E−01 A8 = −8.5911 ×E−02 A10 = 2.6341 × E−02 A12 = 6.5050 × E−04 Fifth surface K = −9.7657 ×E+01 A4 = −6.2544 × E−02 A6 = 3.0029 × E−02 A8 = −4.5729 × E−03 A10 =−2.4190 × E−04 A12 = 5.3660 × E−05 Sixth surface K = −1.0932 × E+01 A4 =−6.9262 × E−02 A6 = 1.6497 × E−02 A8 = −1.8007 × E−03 A10 = −1.6190 ×E−05 A12 = 1.1347 × E−06

The image-capturing lens in Second Example is a design example whereinaperture stop S is arranged between the first lens L1 and the secondlens L2 as shown in FIG. 2. Each of the first lens L1 and the secondlens L2 is a polyolefin-based plastic lens formed by using the materialwherein particles whose maximum particle edge length is not more than 30nanometers ate dispersed, and third lens L3 is a polycarbonate-basedplastic lens containing no particles.

Changes of refractive index caused by temperatures are shown in Table 8.According to this, an amount of changes (ΔfB) of back focus in the caseof temperature rise of +30° C. from the normal temperature (20° C.) is+0.014 mm when A=−8×10⁻⁵/° C. holds for the first lens L1 and the secondlens L2, and it is +0.008 mm when A=−6×10⁻⁵/° C. holds for the firstlens L1 and the second lens L2, while, an amount of changes (ΔfB) ofback focus in the case of temperature fall of −30° C. is −0.014 mm whenA=−8×10⁻⁵/° C. holds for the first lens L1 and the second lens L2, andit is −0.008 mm when A=−6×10⁻⁵/° C. holds for the first lens L1 and thesecond lens L2. TABLE 8 Refractive Refractive Refractive index at indexat index at normal normal normal temperature + temperature − A[/° C.]temperature 30° C. 30° C. First −8 × 10⁻⁵ 1.5318 1.5294 1.5342 lens −6 ×10⁻⁵ 1.5300 1.5336 Second −8 × 10⁻⁵ 1.5318 1.5294 1.5342 lens −6 × 10⁻⁵1.5300 1.5336 Third −14 × 10⁻⁵  1.5830 1.5788 1.5872 lens

THIRD EXAMPLE

FIG. 3 is a sectional view of an image-capturing lens of Third Example.Tables 9 and 10 show lens data related to the present example. TABLE 9(Example 3) f = 5.309 mm   fB = 0.511 mm   F = 2.88   2Y = 6.48 mmSurface No. R(mm) D(mm) Nd μd Aperture ∞ 0.00 1 3.227 1.27 1.69680 55.52 −87.050 0.44 3 −3.364 1.40 1.52500 56.0 4 −1.626 0.35 5 −1.021 0.901.58300 30.0 6 −2.147 0.10 7 2.462 1.10 1.52500 56.0 8 2.283 1.00 9 ∞0.30 1.51633 64.1 10  ∞

TABLE 10 Aspheric surface coefficient Third surface K = −3.69470 × E+00A4 = −2.00408 × E−02 A6 = 5.93561 × E−03 A8 = 5.22016 × E−04 A10 =−2.38137 × E−04 Fourth surface K = −8.46375 × E−01 A4 = −2.02564 × E−02A6 = 1.62756 × E−02 A8 = −4.14965 × E−03 A10 = 6.66591 × E−04 Fifthsurface K = −8.10560 × E−01 A4 = 6.31710 × E−02 A6 = 4.14530 × E−04 A8 =4.30470 × E−03 A10 = −2.38210 × E−03 A12 = 3.81300 × E−04 Sixth surfaceK = −4.69690 × E−01 A4 = 1.50160 × E−02 A6 = 9.94400 × E−03 A8 =−2.33050 × E−03 A10 = 3.92580 × E−04 A12 = −2.86340 × E−05 Seventhsurface K = −8.06986 × E+00 A4 = −1.22203 × E−02 A6 = −1.10253 × E−03 A8= 2.97022 × E−04 A10 = −1.61617 × E−05 A12 = −1.33104 × E−06 Eighthsurface K = −4.95420 × E+00 A4 = −1.49047 × E−02 A6 = 7.29589 × E−04 A8= −2.84963 × E−04 A10 = 4.02284 × E−05 A12 = −2.14994 × E−06

The image-capturing lens in Third Example is a design example whereinaperture stop S is arranged to be closest to an object, and infrared cutfilter IRCF is arranged to be closest to an image as shown in FIG. 3.The first lens L1 is a glass lens, the second lens L2 is apolyolefin-based plastic lens formed by using the material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed in plastic material, third lens L3 is apolycarbonate-based plastic lens formed by using the material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed in plastic material, and fourth lens L4 ispolyolefin-based plastic lens containing no particles. Incidentally, thesecond lens L2 and the fourth lens L4 are polyolefin-based plasticlenses which are different from those in the First and SecondEmbodiments.

Changes of refractive index caused by temperatures are shown in Table11. According to this, an amount of changes (ΔfB) of back focus in thecase of temperature rise of +30° C. from the normal temperature (20° C.)is −0.002 mm when A=−8×10⁻⁵/° C. holds for the second lens L2 and thethird lens. L3, and it is −0.001 mm when A=−6×10⁻⁵/° C. holds for thesecond lens L2 and the third lens L3, while, an amount of changes (ΔfB)of back focus in the case of temperature fall of −30° C. is +0.002 mmwhen A=−8×10⁻⁵/° C. holds for the second lens L2 and the third lens L3,and it is +0.001 mm when A=−6×10⁻⁵/° C. holds for the second lens L2 andthe third lens L3. TABLE 11 Refractive Refractive Refractive index atindex at index at normal normal normal temperature + temperature − A[/°C.] temperature 30° C. 30° C. Second −8 × 10⁻⁵ 1.5250 1.5226 1.5274 lens−6 × 10⁻⁵ 1.5232 1.5268 Third lens −8 × 10⁻⁵ 1.5830 1.5806 1.5854 −6 ×10⁻⁵ 1.5812 1.5848 Fourth −12 × 10⁻⁵  1.5250 1.5214 1.5286 lens

In this case, the amount of changes (ΔfB) of back focus in the case oftemperature rise is a value obtained by disregarding an influence ofthermal expansion of the plastic lens in the case of temperature riseand an influence of thermal expansion of a lens barrel that holds thelens on the basis of calculation. The reason for the foregoing is thatfluctuations in image point position in the case of temperature changesare mainly resulted from changes in the refractive index of the plasticlens.

If a value of A of each lens is made to be one that cancels thefluctuations of image point positions of the total system including alsothermal expansions of the lens barrel and of plastic lenses, suchstructure is more preferable.

Incidentally, the image-capturing lens of the invention is especiallyeffective for the image-capturing apparatus (image-capturing apparatusof the so-called pan-focus system) that houses therein a solid-stateimage sensor having many pixels and has no autofocusing mechanism.Namely, since the pixel pitch of the small-sized solid-state imagesensor having many pixels is small, the focal depth (generally, a valueobtained through calculation of “±(pixel pitch)×2×(F-number ofimage-capturing lens)”) that is proportional to the pixel pitch issmall, resulting in a narrow permissible range for fluctuations of imagepoint positions in the case of temperature changes. The image-capturingapparatus of the pan-focus system is originally of the system to focuson an object at the hyperfocal distance and thereby to cover a rangefrom the infinity to the closest distance with the depth of field.Therefore, image quality for an object at the infinity or at the closestdistance is somewhat out of focus, compared with image quality of anobject at the hyperfocal distance, and therefore, image quality for theinfinity or the closest distance is extremely deteriorated whenfluctuations are caused in the case of temperature changes, which is notpreferable.

Incidentally, First Example is an example of the image-capturing lensfor the solid-state image sensor of a ⅙ inch type with about 300,000pixels at a pixel pitch of 3.75 μm. Second Example is an example of theimage-capturing lens for the solid-state image sensor of a ¼ inch typewith about 1,000,000 pixels at a pixel pitch of 3.2 μm, and ThirdExample is an example of the image-capturing lens for the solid-stateimage sensor of a 1/2.7 inch type with about 1,000,000 pixels at a pixelpitch of 4.5 μm.

Next, the preferred structure to attain the second object will beexplained as follows.

Since the plastic lens formed by using materials wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed in plastic material is a plastic lens having the positiverefracting power in the zoom lens described in Item 2-1, it is possibleto restrain effectively the changes in image point positions of the zoomlens which are caused by temperature changes by forming the positiveplastic lens by using materials wherein particles whose maximum particleedge length is not more than 30 nanometers are dispersed, in the zoomlens in Item 2-2.

Since the zoom lens has at least two plastic lenses in the zoom lensdescribed in Item 2-1, and the plastic lens having the strongestrefracting power among the plastic lenses is a plastic lens formed byusing a material wherein particles whose maximum particle edge length isnot more than 30 nanometers are dispersed in plastic material, it ispossible to restrain effectively the changes in image point positions ofthe zoom lens caused by the temperature changes, by forming the plasticlens having the strongest refractive index by using a material whereinparticles whose maximum particle edge length is not more than 30nanometers are dispersed, in the zoom lens of Item 2-3.

In the zoom lens described in Item 2-1, the zoom lens described in Item2-1 has at least two plastic lenses, and when hi (i=1, . . . )represents a height at which a paraxial marginal ray of light in thecase of full aperture passes on the surface of each plastic lens on theobject side and Φ_(p)i (i=1, . . . ) represents the refracting power ofeach plastic lens, the plastic lens that makes an absolute value ofhi×Φ_(p)i (i=1, . . . ) to be the maximum value in an optional focallength in the case of magnification change from the wide-angle end tothe telephoto end in the optional focal length, is a plastic lens formedby using a material wherein particles whose maximum particle edge lengthis not more than 30 nanometers are dispersed in a plastic material.

When the ray of light is transmitted through the lens, the greater theproduct of a height of the ray of light and the refracting power is, themore the ray of light is deflected, and an influence of changes inrefractive index in the case of temperature changes grows greater.Therefore, on the surface of each plastic lens closer to an object, andwhen hi (i=1, . . . ) represents a height at which a paraxial marginalray of light in the case of full aperture passes on the surface of eachplastic lens on the object side and Φ_(p)i (i=1, . . . ) represents therefracting power of each plastic lens, it is possible to restraineffectively fluctuations in image point positions of the zoom lenscaused by temperature changes, by forming the plastic lens that makes anabsolute value of hi×Φ_(p)i (i=1, . . . ) to be the maximum in anoptional focal length in the case of magnification change from thewide-angle end to the telephoto end in the optional focal length, byusing a material wherein particles whose maximum particle edge length isnot more than 30 nanometers are dispersed in a plastic material.

The zoom lens in Item 2-5 has at least a plastic lens having thepositive refracting power and a plastic lens having the negativerefracting power in the zoom lens described in Item 2-4. Since it iseasy to add an aspheric surface to a plastic lens, when making a zoomlens having at least a plastic lens having the positive refracting powerand a plastic lens having the negative refracting power, it is possibleto add easily an aspheric surface to each lens and thus, variousaberrations in the total system of the zoom lens can be correctedproperly.

With respect to the zoom lens of Item 2-6, the first lens group of thezoom lens closest to an object, in the zoom lens described in any one ofItem 2-1 to Item 2-5 has at least one plastic lens, and the plastic lensis one formed by using the material wherein particles each having themaximum length of not more than 30 nanometers are dispersed in plasticmaterial, thus, it is possible to prevent that an influence offluctuations in image point positions of the zoom lens caused bytemperature changes is enlarged by the magnifications of the lens groupswhich succeed the first lens group, by using, in the first lens group ofthe zoom lens closest to an object, a plastic lens formed by using thematerial wherein particles each having the maximum length of not morethan 30 nanometers are dispersed in plastic material.

With respect to the zoom lens of Item 2-7, in the zoom lens described inany one of Item 2-1 to Item 2-6, the aforesaid zoom lens has an aperturestop, and at least one of the plastic lens adjoining the aperture stopto be closest to it among the plastic lenses, and the plastic lensadjoining further the plastic lens closest to the aperture stop is aplastic lens formed by using the material wherein particles each havingthe maximum length of not more than 30 nanometers are dispersed inplastic material, thus, it is possible to control effectively thefluctuations of image point positions of the zoom lens resulted fromtemperature changes, by using the material wherein particles each havingthe maximum length of not more than 30 nanometers are dispersed inplastic material for at least one of the plastic lens adjoining theaperture stop to be closest to it and the plastic lens adjoining furtherthe aforesaid plastic lens.

Namely, when there are caused refractive index fluctuations resultedfrom temperature changes on a lens near the aperture stop, image pointpositions are fluctuated and optical efficiencies of the total zoom lensare deteriorated. By using the material mentioned above, however, thedeterioration of the optical efficiencies can be controlled to be small.Further, the lens near the aperture stop can be made small to changeinto a lens that can easily be formed despite a material whereinparticles in a nanometer size are dispersed in a plastic material.

With respect to the zoom lens of Item 2-8, the plastic lens formed byusing a material wherein particles in a size of not more than 30nanometers are dispersed satisfies the following condition, in the zoomlens described in any one Item 2-1 to Item 2-7;|A|<8×10⁻⁵/° C.   (1)wherein, A represents a refractive index change with temperatures whosevalue is shown by the (Numeral 1) stated above.

With respect to the zoom lens of Item 2-9, the plastic lens formed byusing a material wherein particles in a size of not more than 30nanometers are dispersed satisfies the following condition, in the zoomlens described in Item 2-8.|A|<6×10⁻⁵/° C.   (2)

With respect to the zoom lens of Item 2-10, the aforesaid zoom lens hasat least two plastic lenses each being formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material, and the plastic lensesformed by using the material stated above include plastic lenses eachhaving a different value of refractive index change with temperature A.By using the plastic lens formed by using a material wherein particleseach having the maximum length of not more than 30 nanometers aredispersed in plastic material having each different value of refractiveindex change with temperature A, it becomes possible to give properly avalue of A that reduces fluctuations in image point positions resultedmost as the total zoom lens from temperature changes, by considering asize of contribution of fluctuations in image point positions caused bytemperature changes of each lens constituting the zoom lens.

The zoom lens of Item 2-11 is represented by the zoom lens described inany one of Item 2-1 to Item 2-10, wherein the particles are inorganicmaterials.

The zoom lens of Item 2-12 is represented by the zoom lens described inItem 2-11, wherein the inorganic materials are oxides.

The zoom lens of Item 2-13 is represented by the zoom lens described inItem 2-12, wherein the oxides are in the state of saturated oxidation.

The zoom lens of Item 2-14 is represented by the zoom lens described inany one of Item 2-1 to Item 2-13, wherein a volume ratio between theplastic material and the particles dispersed in the plastic material iswithin a range of 9:1-3:2.

Though the volume ratio is 80:20, namely, 4:1 in the aforesaid example,it can be adjusted properly within a range of 90:10 (9:1)-60:40 (3:2).If an amount of the particles is less, exceeding the ratio of 9:1, aneffect to restrain a change by temperature turns out to be smaller,while, when the ratio of 3:2 is exceeded, there is caused a problem inmoldability, which is not preferable. It is more preferable to adjust adegree of refractive index change in the material wherein fine particlesare dispersed in a plastic material, by considering an influence on thetotal zoom lens by temperature changes.

The image-capturing apparatus of Item 2-15 has therein the zoom lensdescribed in any one of Item 2-1 to Item 2-14.

Referring to the drawings, there will be explained in detail theembodiment of the invention for attaining the second object of theinvention to which, however, the invention is not limited.

A target of the invention is to control fluctuations in image pointpositions of a zoom lens resulted from temperature changes to be withinthe focal depth of the zoom lens (generally, a value calculated by(±(pixel pitch)×2×(F-number of image-capturing lens))). By doing this,it is not necessary to provide a complicated mechanism to sense the thentemperature by a temperature sensor and to correct the point of focusingwith a stepping motor by an amount of fluctuation of image pointposition at the time of that temperature, in the case of the modewherein no autofocusing is used and a focus is fixed at the hyperfocaldistance, which makes downsizing and weight reduction of the totalimage-capturing apparatus possible.

FIG. 4 is a sectional view in the optical axis direction of animage-capturing apparatus including zoom lenses in Examples 4 and 5. InFIG. 4, the zoom lens includes, in the order from the part of an object,first lens L1, second lens L2, aperture stop S, third lens L3, fourthlens L4, fifth lens L5 and sixth lens L6 (L1-L2 constitute first lensgroup G1, S-L5 constitute second lens group G2 and L6 constitutes thirdlens group G3), and this zoom lens, optical filter F composed of aninfrared cut filter arranged on the zoom lens to be closer to an objectand of a lowpass filter, and solid-state image sensor IS such as CMOS orCCD constitute an image-capturing apparatus. An optical image that isformed on image-capturing surface I by passing of light through the zoomlens and filter F, and through cover glass of solid-state image sensorIS (parallel flat plate) CG is subjected to photoelectric conversion bythe solid-state image sensor IS and is further subjected to prescribedprocessing to be converted into image signals.

Now, the preferred example for the present embodiment will be explainedas follows. Incidentally, in all examples which will be described later,a solid-state image sensor of a 1/3.2 inch type having a pixel pitch of2.8 μm and 2,000,000 pixels is used, and a zoom lens with F-number ofabout 2.88 at the wide-angle end and about 5.0 at the telephoto end isassumed.

Symbols used for respective examples are as follows.

f: Focal length of the total zoom lens

F: F-number

2Y: Length of diagonal line of rectangular effective pixel area ofimage-capturing surface of solid-state image sensor

R: Curvature radius of refractive surface

D: Distance between refractive surfaces on axis

Nd: Refractive index of lens material for d line at a normal temperature

υd: Abbe's number of lens material

Further, “*” in the Surface No. represents an aspheric surface that isshown by the following “Numeral 3” under the condition that the originis represented by the vertex of the surface and the horizontal axis istaken in the optical axis direction, and h represents a height in thedirection perpendicular to the optical axis; $\begin{matrix}{X = {\frac{h^{2}/R}{1 + \sqrt{1 - {( {1 + K} ){h^{2}/R^{2}}}}} + {\sum{A_{i}h^{i}}}}} & ( {{Numeral}\quad 3} )\end{matrix}$wherein, symbols are as follows.

Ai: i^(th) order aspheric surface coefficient

R: Radius of curvature

K: Conic constant

EXAMPLE 4

Lens data of a zoom lens in Example 4 are shown in Table 12.Incidentally, from now on, the exponentiation of 10 (for example,2.5×10⁻³) is to be expressed by using E (for example, 2.5×E-3). TABLE 12(Example 4, Example 5) f = 4.78 to 13.68 mm F = 2.88 to 4.92 2Y = 5.60mm Surface No. R(mm) D(mm) Nd νd 1 46.492 0.80 1.77250 49.6 2 5.832 2.74 3* 39.262 1.95 1.60700 27.0  4* −29.352 (Variable) Aperture ∞ 0.40 57.593 1.60 1.58913 61.2 6 −20.071 0.20  7* 4.138 1.73 1.49700 56.0 836.672 0.90 1.60700 27.0  9* 2.671 (Variable) 10  12.124 1.60 1.5250056.0 11* −23.122 (Variable) 12  ∞ 1.46 1.54880 67.0 13  ∞ 0.40 14  ∞0.50 1.51633 64.1 15  ∞ Variable distance f 4.78 8.10 13.68 D4 17.397.13 1.40 D9 3.80 6.82 12.47  D11 1.40 1.60 1.55 Aspheric surfacecoefficient Third surface K = 1.60490 × E+00 A4 = −5.67770 × E−05 A6 =−4.19810 × E−06 A8 = 6.83470 × E−07 A10 = −1.83210 × E−08 A12 = 2.97430× E−10 Fourth surface K = −2.79810 × E+00 A4 = −4.99660 × E−04 A6 =5.87250 × E−06 A8 = −6.08080 × E−07 A10 = 2.92670 × E−08 A12 = −7.05600× E−10 Seventh surface K = −8.35590 × E−01 A4 = 4.91930 × E−04 A6 =−1.19680 × E−05 A8 = −4.67250 × E−06 A10 = 2.54980 × E−07 Ninth surfaceK = −5.17430 × E−01 A4 = 1.57740 × E−03 A6 = 6.18060 × E−05 A8 =−3.41710 × E−05 A10 = 2.24810 × E−06 Eleventh surface K = −2.67400 ×E+01 A4 = −2.78870 × E−04 A6 = 9.59510 × E−06 A8 = −2.96570 × E−06 A10 =1.82460 × E−07*Aspheric surface

In FIG. 4, a polyester-based plastic lens that is formed by using thematerial wherein particles each having the maximum length of not morethan 30 nanometers are dispersed in plastic material and has refractiveindex change with temperature A=−6×10⁻⁵ is used, as the second lens L2,an acryl-based plastic lens that is formed by using the material whereinparticles each having the maximum length of not more than 30 nanometersare dispersed in plastic material and has refractive index change withtemperature A=−6×10⁻⁵ is used, as the fourth lens L4, a polyester-basedplastic lens that is formed by using the material wherein particles eachhaving the maximum length of not more than 30 nanometers are dispersedin plastic material and has refractive index change with temperatureA=−8×10⁻⁵ is used, as the fifth lens L5, an polyolefin-based plasticlens containing no particles is used as the sixth lens L6, and a glasslens is used as each of lens L1 and lens L3 other than the foregoing.

In the present example, an absolute value of the product of height hiwhere axial marginal ray in the case of an open aperture on the surfaceof the plastic lens closer to an object passes and refracting power ofthe plastic lens Φpi is maximum in the fifth lens L5, and this fifthlens L5 is made to be a plastic lens formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material. Further, an absolute valueof hi×Φpi at that time is 0.695.

Changes of refractive index nd by temperature changes are shown in Table13. Further, Table 14 shows an amount of changes (Δ: fBw, ΔfBT) of backfocus of each of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.) and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 13 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. *Second −6 × 10⁻⁵ 1.6070 1.6052 1.6088 lens *Fourth −6 ×10⁻⁵ 1.4970 1.4952 1.4988 lens *Fifth lens −8 × 10⁻⁵ 1.6070 1.60461.6094 Sixth lens −12 × 10⁻⁵  1.5250 1.5214 1.5286*Plastic lens formed by using the material wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed

TABLE 14 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.008+0.024 −0.008 −0.024

Now, for the purpose of comparison, Table 15 and Table 16 show changesof refractive index nd by temperature changes in the case where allplastic lenses are made to be a plastic lens that does not contain theaforesaid particles an amount of changes (ΔfBw, ΔfBT) of back focus ofeach of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.), and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 15 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. Second −14 × 10⁻⁵ 1.6070 1.6027 1.6113 lens Fourth −12 ×10⁻⁵ 1.4970 1.4934 1.5006 lens Fifth lens −14 × 10⁻⁵ 1.6070 1.60271.6113 Sixth lens −12 × 10⁻⁵ 1.5250 1.5214 1.5286

TABLE 16 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.021+0.090 −0.021 −0.090

When Table 14 is compared with Table 16, it is understood that ΔfBw andΔfBT in the present example are reduced sharply to be controlled withina focal depth.

EXAMPLE 5

Forms and arrangements of respective optical factors in the zoom lens inExample 5 are exactly the same as those in Example 4 (see FIG. 4), andlens data for them are the same as those shown in Table 12 accordingly.However, a difference is in a material of the plastic lens formed byusing the material wherein particles each having the maximum length ofnot more than 30 nanometers are dispersed in plastic material.

More specifically, a polyester-based plastic lens that is formed byusing the material wherein particles each having the maximum length ofnot more than 30 nanometers are dispersed in plastic material and hasrefractive index change with temperature A=−4×10⁻⁵ is used as the secondlens L2, an acryl-based plastic lens that is formed by using thematerial wherein particles each having the maximum length of not morethan 30 nanometers are dispersed in plastic material and has refractiveindex change with temperature A=−4×10⁻⁵ is used as the fourth lens L4, apolyester-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−6×10⁻⁵ is used as the fifth lens L5, anpolyolefin-based plastic lens containing no particles is used as thesixth lens L6, and a glass lens is used as each of lens L1 and lens L3other than the foregoing.

In the present example, an absolute value of the product of height hiwhere axial marginal ray in the case of an open aperture on the surfaceof the plastic lens closer to an object passes and refracting power ofthe plastic lens Φpi is maximum in the fifth lens L5, and this fifthlens L5 is made to be a plastic lens formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material. Further, an absolute valueof hi×Φpi at that time is 0.695.

Changes of refractive index nd by temperature changes are shown in Table17. Further, Table 18 shows an amount of changes (ΔfBw, ΔfBT) of backfocus of each of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.) and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 17 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. *Second −4 × 10⁻⁵ 1.6070 1.6058 1.6082 lens *Fourth −4 ×10⁻⁵ 1.4970 1.4958 1.4982 lens *Fifth lens −6 × 10⁻⁵ 1.6070 1.60521.6088 Sixth lens −12 × 10⁻⁵  1.5250 1.5214 1.5286*Plastic lens formed by using the material wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed

TABLE 18 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.003+0.005 −0.003 −0.005

When Table 16 is compared with Table 18, it is understood that ΔfBw andΔfBT in the present example are reduced sharply to be controlled withina focal depth. It is understood that ΔfBT is especially correctedproperly.

EXAMPLE 6

Lens data of a zoom lens in Example 6 are shown in Table 19. TABLE 19(Example 6, Example 7) f = 5.10 to 14.73 mm F = 2.88 to 5.03 2Y = 5.60mm Surface No. R (mm) D (mm) Nd νd 1 36.264 0.80 1.83481 42.7 2 5.7842.20  3* 33.971 1.00 1.49700 56.0 4 14.450 1.80 1.60700 27.0  5* −41.485(Variable) Aperture ∞ 0.35 6 10.975 1.50 1.58913 61.2 7 −21.312 0.20  8*4.227 2.10 1.52500 56.0 9 −20.068 0.20 10  −38.713 0.90 1.60700 27.0 11*2.889 (Variable) 12  15.309 1.65 1.52500 56.0 13* −15.080 (Variable) 14 ∞ 1.15 1.54880 67.0 15  ∞ 0.40 16  ∞ 0.50 1.51633 64.1 17  ∞ Variabledistance f 5.10 8.63 14.73 D5  13.80 5.83 1.00 D11 3.35 7.19 13.64 D131.80 1.73 1.70 Aspheric surface coefficient Third surface K = 1.84050 ×E+00 A4 = 1.72910 × E−04 A6 = −3.12330 × E−05 A8 = 2.71810 × E−06 A10 =−2.00580 × E−08 A12 = −1.56340 × E−09 Fifth surface K = −4.36820 × E+00A4 = −3.40010 × E−04 A6 = −1.82940 × E−05 A8 = 1.82810 × E−06 A10 =−3.44330 × E−08 A12 = −1.21030 × E−09 Eighth surface K = −4.14420 × E+00A4 = 6.23680 × E−03 A6 = −4.36780 × E−04 A8 = 3.47470 × E−05 A10 =−1.52680 × E−06 Eleventh surface K = −1.75020 × E+00 A4 = 9.18890 × E−03A6 = 3.62270 × E−05 A8 = −8.43350 × E−06 A10 = 2.19960 × E−06 ThirteenthK = 2.02380 × E+00 surface A4 = 2.03460 × E−04 A6 = −1.78390 × E−05 A8 =1.18680 × E−06 A10 = −4.47980 × E−08*Aspheric surface

FIG. 5 is a sectional view in the optical axis direction of animage-capturing apparatus including zoom lenses in Example 6. In FIG. 5,the zoom lens includes, in the order from the part of an object, firstlens L1, second lens L2, third lens L3, aperture stop S, fourth lens L4,fifth lens L5 and sixth lens L6 (L1-L3 constitute first lens group G1,S-L6 constitute second lens group G2 and L7 constitutes third lens groupG3), and this zoom lens, optical filter F composed of an infrared cutfilter arranged on the zoom lens to be closer to an object and of alowpass filter, and solid-state image sensor IS such as CMOS or CCDconstitute an image-capturing apparatus. An optical image that is formedon image-capturing surface I by passing of light through the zoom lensand filter F, and through cover glass of solid-state image sensor IS(parallel flat plate) CG is subjected to photoelectric conversion by thesolid-state image sensor IS and is further subjected to prescribedprocessing to be converted into image signals.

Now, an acryl-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−6×10⁻⁵ is used as the second lens L2, apolyester-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−8×10⁻⁵ is used as the third lens L3, apolyolefin-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−8×10⁻⁵ is used as the fifth lens L5, apolyester-based plastic lens containing no particles is used as thesixth lens L6, a polyolefin-based plastic lens containing no particlesis used as the seventh lens L7 and a glass lens is used as each of lensL1 and lens L4 other than the foregoing.

In the present example, an absolute value of the product of height hiwhere axial marginal ray in the case of an open aperture on the surfaceof the plastic lens closer to an object passes and refracting power ofthe plastic lens Φpi is maximum in the sixth lens L6, and this sixthlens L6 is made to be a plastic lens containing no particles. Further,an absolute value of hi×Φpi at that time is 0.783.

Changes of refractive index nd by temperature changes are shown in Table20. Further, Table 21 shows an amount of changes (ΔfBw, ΔfBT) of backfocus of each of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.) and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 20 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. *Second −6 × 10⁻⁵ 1.4970 1.4952 1.4988 lens *Third −8 ×10⁻⁵ 1.6070 1.6046 1.6094 lens *Fifth lens −8 × 10⁻⁵ 1.5250 1.52261.5274 Sixth lens −14 × 10⁻⁵  1.6070 1.6027 1.6113 Seventh −12 × 10⁻⁵ 1.5250 1.5214 1.5286 lens*Plastic lens formed by using the material wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed

TABLE 21 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.001−0.007 −0.002 +0.007

Now, for the purpose of comparison, Table 22 and Table 23 show changesof refractive index nd by temperature changes in the case where allplastic lenses are made to be a plastic lens that does not contain theaforesaid particles, an amount of changes (ΔfBw, ΔfBT) of back focus ofeach of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.), and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 22 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. Second −12 × 10⁻⁵ 1.4970 1.4934 1.5006 lens Third lens −14× 10⁻⁵ 1.6070 1.6027 1.6113 Fifth lens −12 × 10⁻⁵ 1.5250 1.5214 1.5286Sixth lens −14 × 10⁻⁵ 1.6070 1.6027 1.6113 Seventh −12 × 10⁻⁵ 1.52501.5214 1.5286 lens

TABLE 23 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.045+0.171 −0.047 −0.171

When Table 21 is compared with Table 23, it is understood that ΔfBw andΔfBT in the present example are reduced sharply to be controlled withina focal depth.

EXAMPLE 7

FIG. 6 is a sectional view in the optical axis direction of animage-capturing apparatus including zoom lenses in Example 7. In FIG. 6,the zoom lens includes, in the order from the part of an object, firstlens L1, second lens L2, third lens L3, aperture stop S, fourth lens L4,fifth lens L5 an sixth lens L6 (L1-L3 constitute first lens group G1,S-L6 constitute second lens group G2 and L7 constitutes third lens groupG3), and this zoom lens, optical filter F composed of an infrared cutfilter arranged on the zoom lens to be closer to an object and of alowpass filter, and solid-state image sensor IS such as CMOS or CCDconstitute an image-capturing apparatus. An optical image that is formedon image-capturing surface I by passing of light through the zoom lensand filter F, and through cover glass of solid-state image sensor IS(parallel flat plate) CG is subjected to photoelectric conversion by thesolid-state image sensor IS and is further subjected to prescribedprocessing to be converted into image signals.

A form and an arrangement of each optical factor in the zoom lens inExample 7 are exactly the same as those in Example 6 (see FIG. 5), andlens data thereof are the same as those shown in Table 19. However, amaterial of a plastic lens formed by using the material whereinparticles each having the maximum length of not more than 30 nanometersare dispersed in plastic material (sixth lens L6) is different.

More specifically, an acryl-based plastic lens that is formed by usingthe material wherein particles each having the maximum length of notmore than 30 nanometers are dispersed in plastic material and hasrefractive index change with temperature A=−8×10⁻⁵ is used as the secondlens L2, a polyester-based plastic lens that is formed by using thematerial wherein particles each having the maximum length of not morethan 30 nanometers are dispersed in plastic material and has refractiveindex change with temperature A=−4×10⁻⁵ is used as the third lens L3, apolyolefin-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−4×10⁻⁵ is used as the fifth lens L5, apolyester-based plastic lens that is formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material and has refractive indexchange with temperature A=−6×10⁻⁵ is used as the sixth lens L6, anpolyolefin-based plastic lens containing no particles is used as theseventh lens L7, and a glass lens is used as each of lens L1 and lens L4other than the foregoing.

In the present example, an absolute value of the product of height hiwhere axial marginal ray in the case of an open aperture on the surfaceof the plastic lens closer to an object passes and refracting power ofthe plastic lens Φpi is maximum in the sixth lens L6, and this sixthlens L6 is made to be a plastic lens formed by using the materialwherein particles each having the maximum length of not more than 30nanometers are dispersed in plastic material. Further, an absolute valueof hi×Φpi at that time is 0.783.

Changes of refractive index nd by temperature changes are shown in Table24. Further, Table 25 shows an amount of changes (ΔfBw, ΔfBT) of backfocus of each of the wide-angle end and the telephoto end in the case oftemperature rise of +30° C. for the normal temperature (20° C.) and anamount of changes (ΔfBw, ΔfBT) of back focus of each of the wide-angleend and the telephoto end in the case of temperature fall of −30° C.TABLE 24 Refractive Refractive Refractive index at index at index atnormal normal normal temperature + temperature − A[/° C.] temperature30° C. 30° C. *Second −8 × 10⁻⁵ 1.4970 1.4946 1.4994 lens *Third −4 ×10⁻⁵ 1.6070 1.6058 1.6082 lens *Fifth lens −4 × 10⁻⁵ 1.5250 1.52381.5262 *Sixth −6 × 10⁻⁵ 1.6070 1.6052 1.6088 lens Seventh −12 × 10⁻⁵ 1.5250 1.5214 1.5286 lens*Plastic lens formed by using the material wherein particles whosemaximum particle edge length is not more than 30 nanometers aredispersed

TABLE 25 +30° C. −30° C. ΔfBw[mm] ΔfBT[mm] ΔfBw[mm] ΔfBT[mm] +0.008+0.002 −0.010 −0.002

When Table 23 is compared with Table 25, it is understood that ΔfBw andΔfBT in the present example are reduced sharply to be controlled withina focal depth.

In this case, the amount of changes (ΔfBw, ΔfBT) of back focus in thecase of temperature rise is a value obtained by disregarding aninfluence of thermal expansion of the plastic lens in the case oftemperature rise and an influence of thermal expansion of a lens barrelthat holds the lens on the basis of calculation. The reason for theforegoing is that fluctuations in image point position in the case oftemperature changes are mainly resulted from changes in the refractiveindex of the plastic lens.

If a value of A of the first lens is made to be one that cancels thefluctuations of image point positions of the total system including alsothermal expansions of the lens barrel and of plastic lenses, suchstructure is more preferable. Further, all plastic lenses can naturallybe formed by using the material wherein particles whose maximum particleedge length is not more than 30 nanometers are dispersed in plasticmaterial.

The invention has so far been explained, referring to the embodiments towhich, however, the invention is not limited, and modification andimprovement can naturally be made. An image-capturing apparatus of theinvention is preferably housed in a small-sized digital still camera andin a mobile terminal such as a cellphone and PDA, and it can also beused for other application such as a personal computer camera.

1. An image-capturing lens, comprising: at least two lenses including apositive plastic lens having the strongest refractive power among the atleast two lenses, the positive plastic lens being formed with use of amaterial in which particles each having the maximum length of 30nanometer or less are dispersed in a plastic material. 2-4. (canceled)5. The image-capturing lens of claim 1, wherein the image-capturing lenscomprises the positive plastic lens and a negative plastic lens.
 6. Theimage-capturing lens of claim 1, wherein the image-capturing lensconsists essentially of plastic lenses and at least one of the plasticlenses is the positive plastic lens.
 7. The image-capturing lens ofclaim 1, wherein the image-capturing lens comprises at least one glasslens.
 8. The image-capturing lens of claim 1, wherein an aperture stopis provided and at least one of a plastic lens that is closest to theaperture stop among plastic lenses and a plastic lens that is adjacentto the plastic lens that is closest to the aperture stop is the positiveplastic lens.
 9. The image-capturing lens of claim 1, wherein thepositive plastic lens satisfies the following formula:|A|<8×10⁻⁵/° C. where A represents a change of refractive index causedby a temperature change which is expressed by the following expression:$A = {\frac{( {n^{2} + 2} )( {n^{2} - 1} )}{6n}\{ ( {{{- 3}\quad\alpha} + {\frac{1}{\lbrack R\rbrack}\frac{\partial\lbrack R\rbrack}{\partial t}}} ) \}}$α: Coefficient of linear expansion, [R]: Molecular refraction.
 10. Theimage-capturing lens of claim 9, wherein the positive plastic lenssatisfies the following formula:|A|<6×10⁻⁵/° C.
 11. The image-capturing lens of claim 1, wherein theparticles are inorganic materials.
 12. The image-capturing lens of claim11, wherein the inorganic materials are oxides.
 13. The image-capturinglens of claim 12, wherein the oxides are in the state of saturatedoxidation.
 14. The image-capturing lens of claim 1, wherein a volumeratio between the plastic material and the particles dispersed in theplastic material is within a range of 9:1-3:2.
 15. An image capturingapparatus, comprising: the image-capturing lens described in claim 1.16. A zoom lens, comprising: plural lens groups, wherein magnificationis varied by changing a distance between the lens groups; wherein thezoom lens includes at least two lenses including a positive plastic lenshaving the strongest refractive power among the at least two lenses, thepositive plastic lens being formed with use of a material in whichparticles each having the maximum length of 30 nanometer or less aredispersed in a plastic material.
 17. The zoom lens of claim 16, whereinwhen hi (i=1, . . . ) represents a height at which a paraxial marginalray of light in the case of full aperture passes on the surface of eachplastic lens on the object side and Φ_(p)i (i=1, . . . ) represents therefracting power of each plastic lens, the plastic lens having a highestabsolute value of hi×Φ_(p)i (i=1, . . . ) in an optional focal length inthe case of magnification change from the wide-angle end to thetelephoto end in the optional focal length, is the positive plasticlens.
 18. The zoom lens of claim 16, wherein the first lens group of thezoom lens closest to an object in the zoom lens comprises at least oneplastic lens which is the positive plastic lens.
 19. The zoom lens ofclaim 16, wherein the zoom lens comprises at least two plastic lenseseach being formed with use of the material wherein particles each havingthe maximum length of not more than 30 nanometers are dispersed inplastic material and the at least two plastic lenses each having adifferent value A of refractive index change with temperature.
 20. Animage-capturing lens, comprising: a plastic lens formed with use of amaterial in which particles each having the maximum length of 30nanometer or less are dispersed in a plastic material, wherein a volumeratio between the plastic material and the particles dispersed in theplastic material is within a range of 9:1-3:2.